A location register is a typical component of a control node such as MME (Mobility Management Entity), SGSN (Serving GPRS Support Node), or MSC (Mobile Switching Centre). A subscriber using a UE (user equipment) attaches to a control node (or multiple control nodes simultaneously) via a radio access network. When attaching for the first time, the location registers inside the control node(s) establish a subscriber registration entry for that subscriber.
The subscriber registration entry logs the current status of the subscriber in respect of her attachment. So if a registration status is “attached”, the subscriber is reachable via the radio access network. Typically, this attachment status is confirmed periodically via mobility procedures such as tracking area update, routing area update, or location area update. If such periodic update is not received, the attachment status is automatically changed to implicitly detach. So it is assumed that the subscriber is not reachable anymore. This may happen if the UE loses radio contact for a longer period, for example if the UE is placed in a location without radio coverage (cellar, remote area), or if the battery is suddenly removed. During the period of implicitly detach, the subscriber registration entry is retained, in order to be prepared if radio coverage is suddenly restored.
If, however, the subscriber and her related UE leave the area controlled by the control node while having no radio contact, the subscriber registration entry may stay in that location registers inside that control node.
Furthermore, a subscriber may be forced deleted by operation and maintenance commands. This may be done by operation and maintenance commands directed to the control node itself, or by forced deletion in the central subscriber database such as HLR (Home Location Register) or HSS (Home Subscriber Server).
The latest radio technology LTE (Long Term Evolution) does not have support for the traditional CS (circuit-switched) services such as telephony. Support for telephony in LTE is achieved via a function called CSFB (CS-Fallback). CSFB is a procedure for enabling to fallback from a first Radio Access Technology (RAT) such as the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) using LTE to a second RAT such as GERAN or UTRAN for originated and terminated CS telephony calls.
In brief, CSFB permits in an Evolved Packet System (EPS) the provisioning of telephony CS domain services by re-use of CS infrastructure when the UE is served by E-UTRAN. A CSFB-enabled UE, connected to E-UTRAN, may thus use 2nd or 3rd Generation (2G or 3G) technologies such as GERAN or UTRAN to connect to the CS domain.
When the UE is served by LTE via E-UTRAN, the UE cannot directly perform the required mobility procedures towards the MSC. In this case the SGs-interface between the MME and the MSC is used for mobility management procedures between EPS and the CS domain.
A similar mechanism is defined as combined mobility procedures between SGSN and MSC, and the interface is called Gs-interface.
In both scenarios there is the situation that there are two (temporary or also called visited) subscriber location registers, the first one being in the MME/SGSN, the second one being in the MSC, and the subscriber has a subscriber registration entry in both of them. The UE is registered into the MSC via the MME/SGSN.
Since the two subscriber registration entries belong to the same UE, the same subscriber, and the subscriber registration entry in the MSC is generated via the MME/SGSN, it is essential that these two subscriber registration entries are coordinated to show the same registration state. This is needed in order to terminate calls or short messages to the correct interface and to avoid an increase of signalling if the subscriber has changed registration state in one of the two control nodes.
This is however not possible with the mechanisms provided by today's procedures defined for the SGs-interface or Gs-interface, resulting into the problem that in many situations an attachment state mismatch occurs.